During the production of a hot strip, the rolling is usually effected in rolling mills having a plurality of rolling units and rolling processes which are separated from one another. The rolling stock may come from a separate continuous casting device, for example. Each rolling process may proceed in this case in individual reversing stands, or may be effected in a plurality of rolling units which can each be assembled from a plurality of rolling stands. The rolls in these rolling stands are usually driven by drives, the rotational speed of which is predefined by a superordinate control device.
The rolling stock may also be a hot strip which is produced in a continuous process in a combined casting and rolling installation by an upstream continuous casting machine that is arranged, in particular, in-line.
In the case of such rolling mills, there is the problem that it is difficult in drive terms to precisely distinguish between the drive power which is required to deform the rolling stock and the drive power which is required for applying a strip tension and for conveying the hot strip. A rolling stand therefore cannot be operated as a drive element with controlled strip tension. The mass flow between successive rolling relays has to be decoupled.
In known installations for producing strip steel (conventional wide hot strip trains), this decoupling can be realized between two rolling relays in such a way that the fed material is divided into slabs, and the distances between the individual rolling relays (roughing stands and finishing train) are selected in such a way that both rolling relays are never in engagement with the same roughed strip at the same time. However, this results in a large structural length of the installation, causing high investment costs and thermal losses. As an alternative to this, the roughed strip split into pieces can also be coiled and uncoiled again in devices provided specifically for this purpose, but this is likewise associated with a corresponding outlay.
It is known that a minimum tension control can be effected either by direct tension control or by loop control for coupling two successively engaging rolling units. In both cases, a minimum strip tension is always required for control. However, this minimum required strip tension which is necessary for control and/or is available from tensile measured variables may exceed the yield point of the hot rolling stock to be machined. A material loop which hangs free in parts or entirely also forms an additional strip tension component by virtue of the dead weight. If the sum of the strip tension components locally exceeds the yield point of the rolling stock to be machined, a reduction in the quality and output of the end product produced is unavoidable. Constrictions over the width of the rolling stock and resultant remachining on the end product, in particular, lead to a considerable loss in the ratio between the material used and the material output.
JP 6234613 A has already disclosed an apparatus of the generic type and a method for reducing the strip tension of a rod-shaped long product to a minimum, said method having the following steps:                transporting the rolling stock, by means of a roller table, between two rolling units which are in engagement with the rolling stock, wherein a free-hanging rolling stock loop is formed in a depression arranged in a section of the roller table between the two rolling units;        detecting a measured value of a loop depth of the rolling stock loop by means of a measuring device; and        controlling the main drives of the rolling units by means of a control device taking the measured value of the loop depth into consideration, such that the strip tension is reduced to a minimum.        
As a result of the free-hanging rolling stock loop, however, a not inconsiderable strip tension is introduced into the rolling stock; particularly in the case of hot long products, as arise for example in the continuous production of strip steel in a combined casting and rolling installation, this can lead to constrictions and/or cracks in the strip. It is not clear from said document how the strip tension can be reduced further, or how the loop depth can be set, in particular dynamically for different operating conditions.
To avoid disadvantages of the prior art mentioned above, or else also for plants for continuously producing strip steel in a semi-continuous or continuous multi-stage hot-rolling method in which the roughing stand train and finishing train are connected to one another by a long roughed strip, a satisfactory solution for the decoupling of two successive rolling stages is not known to date.